The present invention relates generally to anchoring devices utilized in subterranean wells and, in an embodiment described herein, more particularly provides a slip having a passageway for lines therethrough and an anchoring device including such slip.
Anchoring devices are generally utilized in subterranean wells to permanently or temporarily fix the location of an assembly relative to a wellbore of the well, and may be fitted with seal elements, such as packer rubbers, to provide a fluid tight seal against the wellbore or other tubular configuration, such as casing, segmented tubing, coiled tubing, liner, or other downhole tool having an inner tubular configuration, when the anchoring device is set. As used herein, the term "wellbore" is used to designate the axially extending bore formed through an earthen formation intersected by a well, as well as other tubular configurations in which an anchoring device may be set, including casing, segmented tubing, coiled tubing, liner, a downhole tool having an axially extending inner bore, etc.
Typical anchoring devices include plugs, packers, liner hangers, tubing hangers, locks, and others. In order to grip the wellbore, which may or may not be lined with protective casing, a typical anchoring device is fitted with one or more elements commonly referred to as slips, which extend radially outward from the anchoring device to bite into, or otherwise grippingly engage, the wellbore when the anchoring device is set. If the anchoring device is temporarily installed, or is intended to be later retrieved from the well, it may also be fitted with a mechanism which retracts the slips out of engagement with the wellbore when desired.
A common type of slip is an individual slip segment. Three or four of these types of slips are usually distributed circumferentially about the exterior of an anchoring device. The slips are typically axially elongated with serrated edges formed on their outside surfaces. Each slip is generally extended and retracted independently of the other slips on the anchoring device, although the same extension and retraction mechanisms may extend and retract all of the slips simultaneously.
When, however, individual slip segments are utilized in an anchoring device which must resist very large loads and/or fluid pressure, several problems with their use become evident. For example, it is difficult to ensure that all of the slip segments extend and grip the wellbore uniformly, so that the anchoring device is centered in the wellbore and is ideally positioned for resisting the loads placed on it. As another example, in order for the slips to adequately grip a wellbore lined with casing, each slip typically must be forced under great contact pressure at a discreet point against the inner surface of the casing, which frequently deforms the casing at that point and prevents subsequent sealing thereto.
Another type of slip used on anchoring devices is known as a barrel slip. This type of slip is typically formed from a tubular piece of material having a serrated or other gripping surface on its outer side surface. In order to make the barrel slip radially deflectable, multiple longitudinal slots are cut partially axially through the tubular material from each of its opposite ends, the slots from each end alternating circumferentially about the tubular material and laterally overlapping each other. In this way, barrel slips permit relatively uniform distribution of gripping force to the wellbore, prevent or reduce damage to casing, and aid in centering the anchoring device within the wellbore.
Unfortunately, typical barrel slips, being circumferentially continuous, do not permit the passage of lines, such as hydraulic, electrical, fiber optic and other control lines, instrument lines, etc., thereacross when an anchoring device is set within a wellbore. Such lines may be passed through a type of slip known as a C-ring slip, however. The C-ring slip is a generally tubular slip with one longitudinal slot extended completely axially through the slip, so that the slip is circumferentially parted and has a generally C-shaped cross-section. It is then possible to route external lines axially through the longitudinal slot.
However, by circumferentially parting a tubular slip to produce a C-ring slip, certain of the disadvantages of individually segmented slips are reintroduced, and certain other disadvantages are added. For example, a C-ring slip does not radially extend or retract uniformly. It is circular in cross-section only when at rest, and will tend to form an elliptical, oval, oblong or other nonuniform shape when radially extended. This results in nonuniform gripping of the wellbore, thereby reducing the load rating of the anchoring device and producing high localized stresses in the wellbore.
As another example, a C-ring slip is typically stressed greatest at a longitudinally extending area radially opposite its longitudinal split, that is, one hundred eighty degrees from the axial slot extending completely through the slip. This greatest stress often occurs at assembly of the anchoring device when the slip is expanded and installed over one or more wedges used to extend the slip, and the slip material yield stress is frequently exceeded, resulting in the slip taking a "set". Such stressing of the slip material will sometimes lead to stress corrosion cracking in service, and enhanced corrosion in the presence of common wellbore fluids, such as hydrogen sulfide.
In order to alleviate these effects of assembling a C-ring slip over the wedges on the anchoring device, some anchoring devices have been produced with helically formed wedges. The slip is likewise manufactured with a complementarily shaped helical inner profile, and the slip is, in essence, threaded onto the anchoring device. However, this increases manufacturing costs and increases assembly complexity, without resolving the problems of overstress of the slip and nonuniform extension and retraction of the slip.
From the foregoing, it can be seen that it would be quite desirable to provide a slip which is capable of uniformly applying a gripping force to casing lining a wellbore, minimizes damage to the casing, permits the passage of lines therethrough, and which may be assembled onto an anchoring device without undue stress and without requiring complex assembly and manufacturing techniques. It is accordingly an object of the present invention to provide such a slip and associated anchoring device.